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Full text of "Text Book Of Mechanical Engineering"

1 1 14                          Appendix VI.

therefore a harmonic curve F. Also the two conductors, being in
serids, will produce twice the voltage of one, and each will cut
the lines twice in a revolution. If n = revs, per sec. and c =
number of conductors round the circle of rotation,

272 "^^C

Average E.M.F. + or -  ==  j- volts.

This apparatus constitutes an elementary Alternating-current
Dynamo or Alternator.

If the conducting cylinder, B, be split as in Fig. 980, the
direction of the current is re-changed at each half-revolution,
making the E.M.F. always of one sign, and the cylinder is then
known as a Commutator, the machine being the elementary form
of the Direct- or Continuous-current Dynamo. Also the
number of splits will correspond with the number of conductors'
or conductor coils. The diagram of E.M.F. will be as shewn at
G, if for a pair of conductors, and the more splits there are, the
more uniform will be the voltage, as at j, giving a steady
continuous current In actual dynamos, through necessary
motives of construction, the current is divided into halves when
passing through the armature conductors, and the E.M.F. is
therefore also halved.

Average E.M.F. -          volts.

io8

When the armature is rotated, and a current produced, the
machine is called a Generator, but if a current sent through an
armature causes rotation, a Motor is constituted. The principle
is identical in both cases, for a motor is only a reversed generator,
with some small differences in detail.

Magnet (or Field) windings. There are only two principal
magnet forms in modern work, the horse-shoe K, Fig. 981, and
the multipolar magnet L, the latter being most likely to persist.
The field may be 'excited' (r) by a separate dynamo w at M,
common in alternator work, or (2) by current produced from the
dynamo itself, as in direct-current (D.C.) machines. The latter
method is practised in three different ways : N, where the line
wire passes continuously through the armature and round the
magnet In series, forming a series- wound3 dynamo : R, by